Robust video delivery requires essentially loss-free delivery of video to all the receivers so the decoders can produce outputs without visible artifacts. This applies both for a single receiver in the unicast case and possibly millions of receivers in the multicast case.
Packet networks lose packets due to a number of impairment events, including congestion, link errors, and re-routing events. Individual losses or short burst losses can be adequately repaired with Forward Error Correction (FEC) or selective retransmission techniques, depending on the exact nature of the error and the delay in the network. However, for longer bursts FEC has poor engineering tradeoffs in terms of delay, bandwidth, and complexity, compared to simple stream redundancy (i.e. sending two or more copies of the same stream).
Similarly, selective retransmission is workable only where there is a very short round-trip time between the receivers and the transmitter. In addition, it is difficult and complex to limit the duration of certain outages in packet networks through techniques like MultiProtocol Label Switching (MPLS) or IP Fast ReRoute (FRR).
A number of stream redundancy techniques are possible. These include spatial techniques where copies of the packets are sent over disjoint paths. Stream redundancy can also include temporal techniques where copies of the packets are delayed in time by more than the expected outage duration.
However, each of these techniques in preexisting systems required both different algorithmic structure and different transport encapsulation and encoding, which makes the design and implementation of transmitters and receivers which want to support multiple techniques difficult.